the fundamentals of backscatter radio and rfid … for multi -antenna backscatter rfid at 5.8...

The Fundamentals of Backscatter Radio and RFID Systems

Part II

Part II8. RFID Modulation and Coding

9. RFID System Communication Protocols

10. Spread Spectrum Backscatter RFID

11. Backscatter Radio and RFID Systems Using Multiple Antennas

12. Backscatter RFID: A Look to the Future

13. Q & A

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RFID MODULATION AND CODING

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RFID Modulation and Coding• Backscatter radio has different requirements

than conventional radio links– Must supply power to passive RF tags.

– Must manage spectral efficiency to avoid interference.

– The need for low-complexity RF tags limits modulation and coding options

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Communication Links

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Conventional Link Backscatter Link

Comparable Complexity Complexity

Reader-to-Tag Communication• Modulation

– Double-sideband amplitude shift keying (DSB-ASK)

– Single-sideband ASK (SSB-ASK)

– Phase-reversal ASK (PR-ASK)

• Coding– Pulse-interval encoding (PIE)

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Reader Modulation• Why ASK?

– Must provide power to passive RF tags

– Must be detectable by the RF tag

• OOK – does not provide constant power to the RF tag

• PSK – requires fairly complex receiver

• ASK – can be detected with a simple envelope detector.

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Reader Spectrum Control

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• The frequency bands available to RF tag systems are divided into channels.

• These channels are designated by FCC’s Part 15 regulations.

• 500 kHz is the maximumchannel BW

• If the number of channels is greater than 50, the maximum conducted power is 1W.

Reader Spectrum Control

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• Interference from other readers can easily disrupt backscatter communication.

Reader Spectrum Control• Single interrogator mode

– Gen 2 standard outlines no spectral requirements

• Multiple interrogator mode– Gen 2 standard specifies spectral constraints to limit

interference in the adjacent channels– Usually accomplished by slowing the data rate or

smoothing the edges of the reader waveform

• Dense interrogator mode– Allows tags to be read when all adjacent channels are

occupied– Uses PR-ASK modulation

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Channel Spectrum Mask

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• Multiple interrogator mode– Usually accomplished by slowing the data rate or

smoothing the edges of the reader waveform

Figure 6.6 in “EPC Radio-Frequency Identity Protocols Class-1 Generation-2 UHF RFID Protocol for Communications at 860 MHz – 960 MHz”, Version 1.2.0, available online at http://www.epcglobalinc.org/standards/uhfc1g2/uhfc1g2_1_2_0-standard-20080511.pdf

Channel Spectrum Mask

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• Dense interrogator mode– Uses PR-ASK modulation

Figure 6.7 in “EPC Radio-Frequency Identity Protocols Class-1 Generation-2 UHF RFID Protocol for Communications at 860 MHz – 960 MHz”, Version 1.2.0, available online at http://www.epcglobalinc.org/standards/uhfc1g2/uhfc1g2_1_2_0-standard-20080511.pdf

Reader Modulation• DSB-ASK

– Simple, but not spectrally efficient

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Reader Modulation• SSB-ASK

– More complex – requires an I/Q modulator

– More spectrally efficient

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Reader Modulation• PR-ASK

– Reduces the width of the spectrum

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Based on Figure 4.48 in D. M. Dobkin, The RF in RFID: Passive UHF RFID in Practice. Burlington, MA: Newnes, 2008.

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Pulse Interval Encoding (PIE)• Tari is defined as the

width of the Data 0 symbol.

• Effectively powers a passive RF tag.

• Range of Tari Values: 6.25 µs to 25 µs.– 6.25 µs => 160 kbps– 12.5 µs => 80 kbps– 25 µs => 40 kbps

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Based on Figure 8.31 in D. M. Dobkin, The RF in RFID: Passive UHF RFID in Practice. Burlington, MA: Newnes, 2008.

RF Tag Demodulation• For PIE, tag must be able to detect the pulse width of the

received signal.

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Figure based on Fig. 4, page 1604 from U. Karthaus and M. Fischer, “Fully Integrated Passive UHF RFID Transponder IC with 16.7-μW Minimum RF Input Power,” IEEE Journal of Solid-State Circuits, vol. 38, no. 10, pp. 1602–1608, 2003.

Tag-to-Reader Communication• Modulation

– Usually a binary modulation scheme

– Amplitude shift keying

– Phase shift keying

• Coding– FM0

– Miller-modulated subcarrier

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Backscatter ASK versus PSK

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ASK Backscatter PSK Backscatter

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FM0 Coding

• Phase inverts at the beginning of each new symbol.

• 1 is constant over symbol period.

• 0 has a single phase change during symbol period.

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Miller-Modulated Subcarrier Coding• 1 has a phase transition

• 0 is constant over symbol period

• No phase transition between symbols unless consecutive zeros

• Baseband Miller encoded waveform is multiplied by a square wave

• M square wave transitions per symbol– M = 2, 4, 8

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Based on Figure 8.39 and 8.40 in D. M. Dobkin, The RF in RFID: Passive UHF RFID in Practice. Burlington, MA: Newnes, 2008.

Miller-Modulated Subcarrier Coding

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M = 4 M = 8

These figures are based on figures 8.39 and 8.40 in D. M. Dobkin, The RF in RFID: Passive UHF RFID in Practice. Burlington, MA: Newnes, 2008.

RFID SYSTEM COMMUNICATIONPROTOCOLS

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RFID Communication Protocols• Over the years, there have been several

protocols used by the RFID community.

• Two early standards that were developed (but never officially ratified) are:– EPCglobal Generation 1 Class 0

– EPCglobal Generation 1 Class 1

• ISO 18000-6A and -6B – developed for European use, but -6B was implemented in the U.S. by Intermec (Intellitag).

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RFID Communication Protocols

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• In this section, I want to briefly introduce the EPC Gen 2 Protocol– I have already introduced the Gen 2 protocol

reader and tag modulation, coding, and spectrum management features.

– Now, I want to walk through a reader-to-tag communication session.

• There is much more detail available than will be presented today• D. M. Dobkin, “The RF in RFID: Passive UHF RFID in

Practice,” Burlington, MA : Newnes, 2008.• http://www.epcglobalinc.org

The EPC Gen 2 Protocol• ISO 18000-6C (EPCglobal Class 1 Generation 2)• The Gen 2 protocol was written for passive UHF

backscatter RF tags.• Features

– Reader talk first– Tags can be read, written, and killed in the field.– Flexible data rates– Provides methods for spectral control to avoid

interference– Uses a Slotted-Aloha algorithm to mitigate data

collisions

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Slotted Aloha

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SPREAD SPECTRUM BACKSCATTER RFID

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Spread Spectrum RFID

• What is spread spectrum?– A technique to “spread” the transmitted signal

over a wide frequency spectrum.

– The spread spectrum technique has roots in military applications.

• Harder to detect (low probability-of-intercept) than narrowband signals.

• Harder to jam than narrowband signals.

– Spread Spectrum Techniques• Frequency Hopping

• Direct Sequence

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Frequency Hopping Spread Spectrum• What is it?

– The transmitter and receiver must hop from one frequency to the next in a pseudorandom fashion.

– Requires synchronization of the transmitter and receiver which is easy is most RFID readers since they are collocated.

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Frequency Hopping Spread Spectrum

• Minimum frequency spacing is the smaller of 25 kHz or the 20 dB channel bandwidth.

• Number of channels:– At least 50 if channel BW ≤ 250 kHz

– At least 25 if channel 250 ≤ BW ≤ 500 kHz

• If number of channels less than 50, then P = .25 W, else P = 1 W.

• Must use each channel equally.

• Channel occupancy limited to 0.4 seconds in any 10 second window

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Frequency Hopping Spread Spectrum• Benefits of using frequency hopping for RFID

systems:– Interference mitigation

– Some multipath fading mitigation• S. R. Banerjee, R. Jesme, and R. A. Sainati, “Performance Analysis of Short Range

UHF Propagation as Applicable to Passive RFID,” in 2007 IEEE International Conference on RFID, Gaylord Texan Resort, Grapevine, TX, USA, March 2007, pp. 30–36.

• S. R. Banerjee, R. Jesme, and R. A. Sainati, “Investigation of Spatial and Frequency Diversity for Long Range UHF RFID,” in IEEE Antennas and Propagation Society International Symposium, San Diego, CA, USA, July 2008, pp. 1–4.

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Direct Sequence Spread Spectrum• What is it?

– Spectrum of the data waveform is spread using a pseudorandom noise (PN) code.

– PN code – a code whose second order statistics approximate that of noise.

– Many PN codes have been developed

• Maximal-length codes (m-sequence)

• Many other codes available as well

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Direct Sequence Spread Spectrum• Direct Sequence Upconversion

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Direct Sequence Spread Spectrum

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• Direct Sequence Downconversion

RFID Spread-Spectrum Applications• An RFID reader must have a way to

differentiate the signals coming from multiple tags.– If all tags respond to the interrogator at once, data

collisions will occur and the read attempt will be unsuccessful.

• RFID tags usually use one of two algorithms to control medium access:– Binary tree algorithm

– Slotted Aloha algorithm (Gen 2 RF tags)

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RFID Spread Spectrum Applications• DS Spread Spectrum provides another way to avoid signal

collisions

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G. D. Durgin and A. Rohatgi, “Multi-Antenna RF Tag Measurement System Using Back-Scattered Spread Spectrum,” in Proceedings of the 2008 IEEE International Conference on RFID, Las Vegas, NV, April, 2008, pp. 1-8.

Rohatgi and G. D. Durgin, “Implementation of an Anti-Collision Differential-Offset Spread Spectrum RFID System,” in Proceedings of the IEEE Antennas and Propagation Society International Symposium, 2006, pp. 3501–3504.

RFID Spread Spectrum Applications• Channel sounding:

Designers and researchers often want to know the characteristics of the backscatter channel.

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J. D. Griffin and G. D. Durgin, “Link Envelope Correlation in the Backscatter Channel,” IEEE Communications Letters, vol. 11, no. 9, pp. 735–737, 2007.

RFID Spread Spectrum Applications

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RFID Spread Spectrum Applications

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correlate

=

Evaluate at τ = 0

BACKSCATTER RADIO AND RFIDSYSTEMS USING MULTIPLE ANTENNAS

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Multiple Antennas – Multiple Options• Reader antenna arrays

for beam forming

• Reader antenna arrays for diversity combining

• Multiple antenna RF tags for pinhole diversity gains

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J. D. Griffin and G. D. Durgin, “Multipath Fading Measurements for Multi-Antenna Backscatter RFID at 5.8 GHz,” in Proceedings of the 2009 International IEEE Conference on RFID, Orlando, FL, April, 2009, pp 322-329.

Reader Beam Forming

• Beam forming will increase both the reader transmitter and receiver antenna gains.

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Power-up Link Budget Backscatter Link Budget

Reader Diversity Combining

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Reader Diversity Combining

• Diversity Combining Techniques–Switch combining

–Gain combining• Requires estimates of the channel

• Equal Gain Combining

• Maximal Ratio Combining (MRC)

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Read Range Gain from Diversity

• For a 5% outage probability, using MRC gives a 3 dB diversity gain – an 18% tag range increase.

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Outage ProbabilityNo MRC MRC

1 x 1 x 1 1 x 1 x 2 1 x 2 x 2

0.5 2.2 1.5 1.0

0.1 11 8.7 6.2

0.05 15 12 8.1

0.01 22 18 12

0.005 26 21 14

0.001 33 28 19

N. Karmarkar, ed., Handbook of Smart Antennas for RFID Systems, Wiley, to be published.

Multiple RF Tag Antennas• Using multiple antennas on the RF tag can

give rise to a pinhole diversity gain.

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J. D. Griffin and G. D. Durgin, “Link Envelope Correlation in the Backscatter Channel,” IEEE Communications Letters, vol. 11, no. 9, pp. 735–737, 2007.

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Multiple RF Tag Antennas• M × L × n Product Rayleigh PDF

J. D. Griffin and G. D. Durgin, “Gains for RF Tags Using Multiple Antennas,” IEEE Transactions on Antennas and Propagation, vol. 56, no. 2, pp. 563–570, 2008.

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Fade Margins for the Backscatter Channel

K = -∞ dB K = 0 dB K = 3 dB K = 10 dB

Outage Probability 10% 1% 10% 1% 10% 1% 10% 1%

1x1x1 (ρ = 0) 15 28 14 26 11 22 4.5 8.6

1x2x1 (ρ = 0) 12 23 11 22 10 20 10 20

1x1x1 (ρ = 1) 22 42 20 40 16 34 6.6 13

1x2x1 (ρ = 1) 15 28

All fade margin values are in dB.

Multiple RF Tag Antennas• Using multiple RF tag

antennas provides– Multiple pinholes

– Pinhole diversity –requires no additional reader hardware or signaling scheme change

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– Will not result in more severe fading in LOS links

– Additional sources of power for passive RF tags

High-Frequency Backscatter Testbed

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5.8 GHz Direct Conversion Receiver Boards

5.8 GHz Receiver



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5.8 GHz Dual-AntennaRF Tag (DTAG)

5.8 GHz Single-AntennaRF Tag (STAG)


NLOS Measurement Environment

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STAG and DTAG NLOS Fading

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NLOS DTAG MeasurementNLOS STAG MeasurementJ. D. Griffin and G. D. Durgin, “Multipath Fading Measurements for Multi-Antenna Backscatter RFID at 5.8 GHz,” in Proceedings of the 2009 International IEEE Conference on RFID, Orlando, FL, April, 2009, pp 322-329.

Measured and Derived Fade Margins for Product-Rayleigh Fading Links

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Outage Probability

MeasuredSTAG Analytic

STAG

MeasuredDTAG Analytic

DTAGRX 1 RX 2 RX 1 RX 2

50% 3.4 4.1 4.1 2.6 2.9 2.9

10% 14 16 15 12 12 12

5% 18 20 20 15 14 16

1% 24 27 28 22 23 24

All fade margin values are in dB.J. D. Griffin and G. D. Durgin, “Multipath Fading Measurements for Multi-Antenna Backscatter RFID at 5.8 GHz,” in Proceedings of the 2009 International IEEE Conference on RFID, Orlando, FL, April, 2009 , pp 322-329.

LOS STAG Bistatic Measurements

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Channel power measured at RX 2

Backscatter RFID: A Look to the Future

• Multiple antenna RF tags and readers• Spread-spectrum communication• High RF tag sensitivity using exotic

semiconductorsJ. P. Curty, N. Joehl, C. Dehollain, and M. J. Declercq, “Remotely Powered Addressable UHF RFID Integrated System,” IEEE Journal of Solid-State Circuits, vol. 40, no. 11, pp. 2193–2202, 2005.

• Data coding and encryption• New reader waveforms• Use of higher frequencies• New applications of backscatter radio

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New Reader Waveforms• Passive RF tag range is

currently limited by the voltage that the tag can rectify from the incident wave.

• A voltage multiplier is used to rectify and increase the received voltage

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Matthew Trotter

New Reader Waveforms• The efficiency of a charge pump is

• Use a waveform that increases VAC while still maintaining the average power required by the FCC.

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M. S. Trotter, J. D. Griffin, and G. D. Durgin, “Power Optimized Waveforms for Improving the Range and Reliability of RFID Systems,” in Proceedings of the 2009 International IEEE Conference on RFID, Orlando, FL, April, 2009, pp 80-87.

Existing Gen 2 RF Tags• Experiments have shown that this technique

will work with existing RF tags that comply with the EPC Gen 2 protocol.

• M. S. Trotter, J. D. Griffin, and G. D. Durgin, “Power Optimized Waveforms for Improving the Range and Reliability of RFID Systems,” in Proceedings of the 2009 International IEEE Conference on RFID, Orlando, FL, April, 2009, pp 80-87.

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Use of Higher Frequencies• Most backscatter RF tags (in the U.S.) operate

in the 902-928 MHz frequency band.– This is an unlicensed band for industrial, scientific,

and medical use (ISM band).

– This band falls under Part 15 of the FCC rules.

• Some tags also use the 2400-2483.5 MHz band

• Another frequency band is available at 5725-5850 MHz

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J. D. Griffin and G. D. Durgin, “Complete Link Budgets for Backscatter Radio and RFID Systems,” IEEE Antennas and Propagation Magazine, April, 2009.

Advantages of Higher Frequency

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• Smaller Antennas– Allow multiple antennas to be used on an RF tag

without increasing its footprint

– Allow for compact antenna arrays at the reader

– Easier to use multiple antennas on the reader and RF tag

Advantages of Higher Frequency

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• Increased Antenna Gain– For fixed aperture, antenna gain increase as

wavelength decreases

– As gain increases, so does directivity.

Advantages of Higher Frequency• Increased Object Immunity

– As the frequency increases, the electrical separation between an RF tag antenna and an object will increase

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Advantages of Higher Frequency• More available BW

– The 5725 – 5850 MHz ISM frequency band has 125 MHz of bandwidth compared to 26 MHz in the 902-928 MHz band.

– Will allow for use of spread spectrum techniques

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New Applications Many more applications possible (not just identification) Integration with Sensors and Passive Data Exchange Last-leg of a Personal Area Network (PAN) The “internet of things” Demonstration of wireless MP3 player using backscatter:

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Further ReadingRFID Modulation and CodingD. M. Dobkin, The RF in RFID: Passive UHF RFID in Practice. Burlington, MA: Newnes,

2008.K. Finkenzeller, RFID Handbook: Fundamentals and Applications in Contactless Smart

Cards and Identification, 2nd ed. New York: John Wiley and Son LTD, 2003.EPCTM Radio-Frequency Identity Protocols Class-1 Generation-2 UHF RFID Protocol for

Communications at 860 MHz – 960 MHz, Version 1.2.0, October 23, 2008, [Available:] http://www.epcglobalinc.org/standards/uhfc1g2/uhfc1g2_1_2_0-standard-20080511.pdf (Accessed on May 12, 2009).

U. Karthaus and M. Fischer, “Fully Integrated Passive UHF RFID Transponder IC with 16.7-μW Minimum RF Input Power,” IEEE Journal of Solid-State Circuits, vol. 38, no. 10, pp. 1602–1608, 2003.

RFID System Communication ProtocolsD. M. Dobkin, The RF in RFID: Passive UHF RFID in Practice. Burlington, MA: Newnes,

2008.EPCTM Radio-Frequency Identity Protocols Class-1 Generation-2 UHF RFID Protocol for

Communications at 860 MHz – 960 MHz, Version 1.2.0, October 23, 2008, [Available:] http://www.epcglobalinc.org/standards/uhfc1g2/uhfc1g2_1_2_0-standard-20080511.pdf (Accessed on May 12, 2009).

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http://www.epcglobalinc.org/standards/uhfc1g2/uhfc1g2_1_2_0-standard-20080511.pdf�




Further ReadingSpread Spectrum Backscatter RFIDG. D. Durgin and A. Rohatgi, “Multi-Antenna RF Tag Measurement System Using Back-

Scattered Spread Spectrum,” in Proceedings of the 2008 IEEE International Conference on RFID, Las Vegas, NV, April, 2008, pp. 1-8.

Rohatgi and G. D. Durgin, “Implementation of an Anti-Collision Differential-Offset Spread Spectrum RFID System,” in Proceedings of the IEEE Antennas and Propagation Society International Symposium, 2006, pp. 3501–3504.

Many online spread spectrum tutorials. One such is http://www.sss-mag.com/ss.html#tutorial (accessed 4/16/09).

R. W. Dixon, Spread Spectrum Systems with Commercial Applications, 3rd ed. New York: Wiley Interscience, 1994.

J. G. Proakis, “Digital Communications,” 4th ed. New York: McGraw-Hill, 2001.Frequency Hopping Regulations: Links are provided from the FCC’s Office of

Engineering and Technology website: http://www.fcc.gov/oet/info/rules/

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http://www.fcc.gov/oet/info/rules/�

Further ReadingBackscatter Radio and RFID Systems Using Multiple AntennasJ. D. Griffin and G. D. Durgin, “Gains for RF Tags Using Multiple Antennas,” IEEE

Transactions on Antennas and Propagation, vol. 56, no. 2, pp. 563–570, 2008.J. S. Kim, K. H. Shin, S. M. Park, W. K. Choi, and N. S. Seong, “Polarization and Space

Diversity Antenna Using Inverted-F Antennas for RFID Reader Applications,” Antennas and Wireless Propagation Letters, vol. 5, no. 1, pp. 265–268, 2006.

Rahmati, Z. Lin, M. Hiltunen, and R. Jana, “Reliability Techniques for RFID-Based Object Tracking Applications,” in 37th Annual IEEE/IFIP International Conference on Dependable Systems and Networks (DSN’07), Edinburgh, UK, 2007, pp. 113–118.

J. D. Griffin and G. D. Durgin, “Multipath Fading Measurements for Multi-Antenna Backscatter RFID at 5.8 GHz,” in Proceedings of the 2009 International IEEE Conference on RFID, Orlando, FL, April, 2009, pp. 322-329.

Backscatter Radio: A Look to the FutureM. S. Trotter, J. D. Griffin, and G. D. Durgin, “Power Optimized Waveforms for

Improving the Range and Reliability of RFID Systems,” in Proceedings of the 2009 International IEEE Conference on RFID, Orlando, FL, April, 2009, pp 80-87.


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the fundamentals of backscatter radio and rfid … for multi -antenna backscatter rfid at 5.8...

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